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Old 09-22-13, 05:45 AM   #1
graycenphil
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Power Cuts Out Under Load

Mt first try with an electric bike. I bought a used electric bike on Craigslist - Cheap Walmart mountain bike with a "Sampson High Power" front wheel motor and a 36 volt/ 20 AH battery, plus controller box and thumb operated throttle. My plan was to move the components onto my old Schwinn tandem, which I have done.

It worked fine for the brief ride I took around the parking lot. On longer rides, here is what happens: If you ride at max power for more than 10 or 20 seconds, it cuts out. If you release the throtttle and reapply it, usually it will work. Sometimes, though, particularly if it ihas cut out a few times already, it won't. But if I unplug it, then plug it back in, it will work.

If I ride at partial throttle, it will run for a pretty long time. But if it is enough pwer to help climb a hill, it cuts out pretty quickly. I had the problem both before and after switching it to the Schwinn.

Any idea what's wrong?

Thanks.
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Old 09-22-13, 03:04 PM   #2
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Two possible reasons, if the battery is a Lithium Ion Battery..

1.) Either the BMS on the battery, or the controller itself is limiting the amount of power to the motor.
(This is usually the case when you are on a really heavy bicycle and are trying to pull way too much power out of the battery to go full throttle.)

2.) (More likely) is that the battery has one (or more) bad cells in it and when trying to accelerate hard, it drops the voltage way to low and the BMS or controller trip too easily.


(A 36v 20AH battery, SHOULD easily last 30 miles if going between 15-18mph. If you're planning on trying to go 20+mph on 36v you WILL trip the BMS or controller.)
If the batteries are lead acid, you'll trip the controllers low voltage function when trying to go faster than 18mph, for sure.

Can you supply some pics?
What kind of battery(s)?
How much did you pay?
Do you have a copy of the craigslist post?
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Old 09-22-13, 06:53 PM   #3
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Thanks SpecialX I took pictures of the wheel, battery, thumb control and electronics. I think I can post them.

The battery appears to be a lithium, though Ican't say for sure. It is well sealed up. I paid $120, but don't have the ad anymore. It didn't have any specifics, as I recall.
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Old 09-22-13, 07:19 PM   #4
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Simple answer, You are over using either the battery or the motor and asking too much of it thus it cuts out to save the battery or motor... Or you could have the problem SpecialX described...
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Old 09-22-13, 09:28 PM   #5
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Originally Posted by 350htrr View Post
Simple answer, You are over using either the battery or the motor and asking too much of it thus it cuts out to save the battery or motor... Or you could have the problem SpecialX described...
If you are using this on a tandem with two riders while going uphill, that will draw a lot of power. Would make sense if the battery is dropping below the "low voltage cutoff" in those conditions
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Old 09-23-13, 04:27 AM   #6
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I'm sure the tandem is asking a lot (it asks a lot of me when I pedal it), but it did pretty much the same thing on the original single mountain bike too. Even asking for fiarly modest power, say just enough to feel some assist on a hill, will make it cut out pretty quickly. This would be probably less than "half throttle", but continuously. I should think the system would be able to deliver that? It does have a lot of power when you give it "full throttle" for a brief time.
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Old 09-23-13, 06:48 AM   #7
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Thanks SpecialX I took pictures of the wheel, battery, thumb control and electronics. I think I can post them.

The battery appears to be a lithium, though Ican't say for sure. It is well sealed up. I paid $120, but don't have the ad anymore. It didn't have any specifics, as I recall.
Theres your answer, right there.
$120 for bike, motor, controller and battery.
More than likely, the battery has a bunch of bad cells in it.
The battery alone costs a MIN of $300 brand new. The Ebike kit, itself, costs a min of $200.
Anyone selling all of it for $120 knows that the battery has a few bad cells in it.
Be happy that you paid $120 for it! That's a good deal, even with a bad battery.

Do you have a volt meter to check the battery voltage after charging and after discharging?
Does the battery have an external BMS for you to check the voltages at the cell level?

If you don't have a voltmeter, go buy one at the local harbor freight (like $6 for a cheapy one) or Home Depot / Lowes (again buy a cheapy {but slightly better** $20 one).

A VM is a necessity for any ebike owner.

Last edited by SpecialX; 09-23-13 at 06:55 AM.
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Old 09-23-13, 11:00 AM   #8
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Would a bad cell (or a few) cause this, but still have lots of power for a short time? I do have a voltmeter, and I put the battery on the charger this morning, wondering if a full charge might make a difference. I assume I'm loking for at least 36 volts when it is fully charged; what else should I be testing for?
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Old 09-23-13, 01:19 PM   #9
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Would a bad cell (or a few) cause this, but still have lots of power for a short time? I do have a voltmeter, and I put the battery on the charger this morning, wondering if a full charge might make a difference. I assume I'm loking for at least 36 volts when it is fully charged; what else should I be testing for?
Yes, a bad cell (group) or two would DEFINITELY cause the Low Voltage Cutoff. (LVC)

The voltage on a good fully charged LiFePo4 battery should read 40.8v and above..
After fully charged and run for at least a mile, the voltage should read between 38.4 and 39.6v and should stay in that range for at least 10 miles in.
(These are all while not moving, as voltage under load will be lower.)
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Old 09-23-13, 05:27 PM   #10
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Thanks. I'll check it tomorrow.
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Old 09-24-13, 04:05 PM   #11
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I tested it today, and got erratic and varying readings, from 0 to30 volts, jumping around. So maybe a bad battery? I opened it up looking for a bad connection but didn't find anything obvious. I was surprised that it had a printed circuit board in there too; I'm not used to such a sophisitated battery. I was expecting a positive and a negative terminal, and not much else.

Another thought I had would be to run a battery directly to the motor, maybe a 12 volt or two 12 volts in series with an on/off switch. Can I do this? Will it work, and will it damage the motor?
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Old 09-25-13, 05:22 AM   #12
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Originally Posted by graycenphil View Post
I tested it today, and got erratic and varying readings, from 0 to30 volts, jumping around. So maybe a bad battery? I opened it up looking for a bad connection but didn't find anything obvious. I was surprised that it had a printed circuit board in there too; I'm not used to such a sophisitated battery. I was expecting a positive and a negative terminal, and not much else.

Another thought I had would be to run a battery directly to the motor, maybe a 12 volt or two 12 volts in series with an on/off switch. Can I do this? Will it work, and will it damage the motor?
1.) Battery NEEDS to be at LEAST 38.4 volts to operate the bike properly.
2.) The printed circuit board is a BMS, it stands for battery management system. It is supposed to protect the battery from overcharging and low voltages while riding. (That's why the motor cuts out when you go full throttle.

Did you test the voltage on the line AFTER the BMS (nearer the controller) or BEFORE the BMS?
Were you using an analog or digital volt meter? The voltage shouldn't be "jumping around"..
Some pics of you checking the voltages would help.
Also take a bunch of close up pics of the battery, it's wires and the BMS and show where you are checking the voltages.
There sounds like a lose wire somewhere in the battery or on the BMS. But can't know for sure without pics.

Edit: You would need THREE - 12v batteries in series for 36v (and at least 20AH batteries).

Let's go with pics first.
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Old 09-25-13, 11:42 AM   #13
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I was testing the voltage well after the BMS - using a digital VOM,I inserted the meter leads into the plug which is a couple feet outside of the battery, on the way to the controller. It did not change if I jiggled the battery pack, nor did it stay steady with the pack stable. I did not test the voltage once I opend the battery pack, but I did just now to get the picture. It showed a steady 40.5 volts, and did not change when I moved tha battery pack around. So perhaps I should put it back together and try it out.

My thought with the one or two 12 vot batteries was to give the moter moderate, steady power. I don't know if the controller operates by varying the voltage to the motor, but I thought that if it did this might give me a simple solution to a moderate level of assist.

Thanks for your patience.
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Old 09-25-13, 02:32 PM   #14
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Sounds like th BMS is going bad, if the voltage fluctuates on the wires AFTER the BMS.
When putting it back together, leave the BMS board outside of the main battery so you can test voltages before and after it.
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Old 09-27-13, 09:54 AM   #15
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Okay, put it all back together with the BMS outside the battery box and I got steady voltage readings. 40.4 volts before going for a ride of a little under a mile, including two decent hills. Shortly after the ride 40.0 volts.

Performance on the ride was the same: worked well for a while, then cut out. Switched off and back on and it worked. Cut off faster on the hills. After a while, switching off and on didn't restore power so I had to unplug it and plug it back in to make it go. So it definitely is worse after doing some work.

Just out of curiosity, when I got home I put the front wheel up off the ground and ran the motor. It ran fine at full throttle for 2 or 3 minutes, something it definitely would not have done while riding.
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